Disk drive apparatuses that use various types of recording disks, such as an optical disk, a magneto-optical disk and a flexible magnetic disk, are known. Among them, hard disk drives (HDDs) have been widely used as a computer recording apparatus and one of the recording apparatuses essential for current computer systems. In addition to computer systems, HDDs are increasingly finding application thanks to its excellent characteristics, for example in video image recording/reproducing apparatuses, car navigation systems, mobile phones and as removable memories used in digital cameras.
A magnetic disk used in an HDD has a plurality of concentrically formed data tracks and servo tracks. The servo track has a plurality of servo data blocks (servo patterns) spaced apart from each other in the circumferential direction. User data is recorded in the area between servo sectors. A head element unit in the form of a thin film element accesses a desired area (address) according to the servo data to write or read user data.
Each servo pattern (hereinafter referred to as a product servo pattern) includes a cylinder ID, a sector number and a burst pattern. The cylinder ID represents a track address and the sector number represents a sector address in a track. The burst pattern has positional information on the magnetic head relative to a track.
As described above, each track has product servo patterns in the form of multiple sectors spaced apart from each other in the circumferential direction. Product servo patterns located at the same position in the circumferential direction, that is, having the same sector number, are aligned (in phase) in the circumferential direction throughout the tracks. The product servo pattern is written to the magnetic disk in the factory before shipment of the HDD as a product. In general, product servo patterns have been conventionally written using a servo writer as an external apparatus. After the HDD is mounted on the servo writer and the servo writer positions the head in the HDD by means of a positioner (external positioning mechanism), a product servo pattern generated by a product servo pattern generation circuit is written to the magnetic disk.
Currently, the writing process of the product servo pattern (Servo Track Write: STW) takes up a major part of the HDD manufacturing cost. Particularly, in recent years, competition to increase HDD capacity have heated up, which has accordingly increased TPI (Track Per Inch). When the TPI increases, the number of tracks increases and the track width (track pitch) decreases. These have increased the STW time and driven sophistication of servo writers, thereby contributing to increased STW cost. To reduce the cost, efforts have been made, for example, to reduce the cost of servo writers and the amount of the STW time.
SSW (Self Servo Write) has been proposed as one of approaches to solve the above problem. Unlike conventional STW, SSW uses only the mechanical feature in the HDD body along with an external circuit to control the spindle motor (SPM) and the voice coil motor (VCM) in the HDD so as to write a product servo pattern, thereby reducing the cost of the servo writer.
The SSW takes advantage of the fact that a read element and a write element of the head element unit are disposed at different positions in the radial direction (hereinafter referred to as a read-write offset), so as to use the read element to read patterns already written on the inner or outer diameter side for positioning the head element unit and use the write element to write a new pattern on a desired track separated by the read-write offset. In addition to the product servo pattern, SSW writes other patterns on the recording surface and uses them to perform head position control and timing control.
In general, an HDD has a plurality of recording surfaces, a plurality of head element units corresponding to the recording surfaces and an actuator that supports the plurality of element units. SSW uses one head element unit selected from the plurality of head element units (hereinafter referred to as a propagation head) to read a pattern on a recording surface and uses the signal of the readout pattern to control the actuator, so as to position the plurality of head element units. The positioned head element units simultaneously write patterns on the respective recording surfaces. For example, Japanese Laid-Open Patent No. 2003-338147 (Patent Document 1) discloses a method for determining a track pitch considering that the read-write offset changes depending on the radial position.
When the SSW is used to write a product servo pattern, it is important to write the pattern for each track at a desired track pitch. In one approach, the written pattern is actually read out and the readout is compared with a preset reference value. The track pitch is controlled by controlling the position of the head such that the measured value and the reference value coincide. In this way, each track pitch can coincide with the desired value.
Specifically, the SSW calculates function values of the readout amplitudes of the patterns on three adjacent tracks and moves the head such that the function values coincide with the preset reference value. The reference value can be determined by using a rotary positioner to write an ideal pattern in an HDD of the same design and actually measuring the function value of the pattern.
However, in the actual SSW, there are product-to-product variations. Thus, track pitches controlled based on the reference value have shown that they significantly vary in some areas depending on products. Particularly, patterns sequentially written from the inner diameter side to the outer diameter side of the magnetic disk based on the reference value have shown that track pitches in the outer diameter-side area significantly vary.